The little amount of progestin in the circulation will have a significant impact only on those tissues very sensitive to the female sex steroids, estrogen and progesterone. The contraceptive effect is any more dependent upon endometrial and cervical mucus effects, since gonadotropins are not consistently suppressed. The endometrium involutes and becomes hostile to implantation, and the cervical mucus becomes thick and impermeable. Approximately 40% of patients will ovulate normally. Tubal physiology may also be affected, but this is speculative.

Because of the little dose, the minipill must be taken daily at the same time of day. The
change in the cervical mucus requires 2-4 hours to take effect, and, most importantly, the impermeability diminishes 22 hours after administration.

Ectopic pregnancy is not prevented as effectually as intrauterine pregnancy. Although the overall incidence of ectopic pregnancy is not increased, when pregnancy occurs, the clinician must suspect that it is any more likely to be ectopic.

There are no significant metabolic effects,137 and there is an immediate return to fertility upon discontinuation (unlike the delay seen with the combination oral contraceptive).